Liquid crystal display devices have come into wide use as flat panel displays, taking advantage of their characteristic features such as thinness, lightweight and power saving, and have found an increasingly wide variety of uses for televisions, personal computers, mobile phone terminals such as smartphones, personal digital assistance such as tablet terminals, and the like.
Backlight systems for illuminating a liquid crystal panel from the back face side are in widespread use as a liquid crystal display device, which is equipped with an edge-lit (side-lit) backlight unit, a direct-lit backlight unit or the like. As shown in FIG. 18, an edge-lit backlight unit 101 to be provided in a liquid crystal display device generally includes light sources 102, a light-guiding plate 103 having a rectangular plate-like shape and disposed such that the edge part thereof is aligned with the light sources 102, optical sheets 104 laminated on the front face side of the light-guiding sheet 103, and a reflection sheet 105 disposed on the back face side of the light-guiding sheet 103. The sheets used as the optical sheets 104 include (1) an under-layer light diffusion sheet 106 that is overlaid on the front face side of the light-guiding sheet 103 and primarily has the function of diffusing rays of light, (2) a prism sheet 107 that is overlaid on the front face side of the light diffusion sheet 106 and has the function of reflecting rays of light toward a normal direction, and (3) an upper-layer light diffusion sheet 108 that is overlaid on the front face side of the prism sheet 107 and slightly diffuses rays of light to inhibit the lack in uniformity of luminance caused by, for example, the shapes of protruding prism portions of the prism sheet 107 (see Japanese Unexamined Patent Application, Publication No. 2005-77448).